The invention relates to circuitry for producing a current that is related to the intensity of illumination. One well known use of such circuitry is found in automatic cameras. The camera shutter and/or aperture is automatically operated to expose the film properly. One approach is to convert the illumination into a current (light domain to current domain converter) and the current used to control the camera (a current domain to mechanical domain converter). Silicon photo diodes have been found to have excellent properties for converting light into current. They are very linear in terms of current versus illumination, have the proper light wavelength response (when used with simple optical filters), display no undesired memory characteristics, and respond rapidly to changes in illumination so as to be useful in sensing the light produced by conventional flash units. For example, when the illumination is changing, as in the case of flash photography, use of the current domain makes it easy to record the incident light. A simple electronic circuit can integrate the current and indicate when sufficient light has been received to properly expose the film. It does not matter whether the light intensity varies or is steady so long as the quantity is integrated with respect to time.
Finally, and of great importance, is the fact that silicon photo diodes can be manufactured as an integral part of a silicon integrated circuit (IC). The photo diode is compatible electronically and process wise so that the complete electronic circuit used to operate a camera can be made using only one IC chip, if desired, manufactured with conventional bipolar technology. Two basic problems attend such IC designs. First the photo diode must sense light values that vary over about six orders of magnitude. This would involve viewing sunlit scenes as well as dimly lit rooms and even moonlit outdoor scenes. Second, even though silicon photo diodes are in fact quite linear in responding to low light intensity, their actual current values are at the nanoampere level. In silicon bipolar transistors this is into the range of P-N junction leakage. Thus, it has proven difficult to design bipolar IC configurations that can handle the large dynamic range and still be operative at the low current values.